We propose to study the themes and underlying chemical principles of helix-helix interactions in membranes as an approach to building an understanding of folding, oligomerization, and function. We have suggested that the stability of helical membrane proteins and the oligomeric interfaces between them can be conceptually simplified by considering each helix or helical hairpin in the structure as a separately stable domain in a bilayer environment. The interactions of such domains then describe important components of the folding and oligomerization of these membrane proteins. Alternative states of genomes may be membrane proteins with transmembrane helices, progress leading to predictive power should see widespread application. In our studies of glycophorin A transmembrane domains, we have established that the close fit of van der Walls surfaces provide significant specificity and interaction energy in the interactions of these helices. A simplified preliminary theory accounts for variations in this interface, and has led to redesigns of the structure that will be tested experimentally during the next project period. Measurements of the delta-deltaG associated with sequence changes in the interface. Measurement of the structure and association energy of re-designed glycophorin interface will test our theories and/or allow us to improve them. A genetic screen for helix interactions in E. coli inner membranes has been successfully implemented. Using randomized substitutions in left-and right- handed motifs, large scale searchers for sequences that oligomerize will be conducted, and the motifs found will be studied in terms of structure and energy. Thus, two major lines will be pursued: the identification of sequence motifs in helix interactions, and the continued development of predictive theoretical algorithms. Searches fore drugs that modulate domain interactions or influence oligomerization in soluble proteins have met with extremely limited success. The finding that helix interactions exploit small interaction areas and can have precisely defined interaction surfaces suggests that these interactions in membranes may be a more amenable target. The principles and motifs we hope to find can aid in the development of agents to modulate membrane protein structure and function.